Add support for clang-cl.

[yangbowen]

Previously clang-cl would not be detected as MSVC despite using MSVC command line syntax. This PR adds such support.

In addition, with this PR, on Clang ThinLTO will be used, which hopefully offers some performance benefit.
It appears that clang-cl would ignore the MSVC option for LTO /GL & /LTCG. ThinLTO will also be used on clang-cl.

Additionally, x86_sse option is modified to support SSE4.1, SSE4.2, AVX, AVX2, AVX512.
SSE2 is not unavailable on x86-64. Instead it's always available so compilers enable SSE2 support on x86-64 implicitly.

Various refactorings are made to realize the above changes while keeping it cleanly organized.

[LBPHacker]

I'll go through this a bit later more thoroughly, but yeah, as I think even comments in meson.build say, these sorts of upgrades are much needed and appreciated.

One thing I'm not certain about right away is your removal of SSE explicitly configured but unavailable in msvc targeting 64-bit machines. I'm very much aware that x86-64 mandates at least SSE2, but I vaguely remember the compiler not accepting that switch (read: throwing warnings about it) for the same reason.

[yangbowen]

Well, I tweaked the part where x86_sse_level is translated into compiler flags, so that while the level is 20, the /arch flag will only be added when supported :)

The-Powder-Toy/meson.build

Line 303 in b4bb54c

elif x86_sse_level >= 20 and host_machine.cpu_family() == 'x86'

Perhaps comment should be added that specifically explains this, in case it's not clear?

[yangbowen]

The rationale was that, x86_sse_level would always reflect whatever instruction set that's in use, instead of what compiler flags to specify (since the latter needs separate treatment anyway)

[LBPHacker]

Yep, lgtm, I'll sort out a few problems we have with some pipelines so we don't immediately start looking at this PR when eventually something fails horribly (our ghactions setup doesn't run all jobs all the time, only the ones we consider more important, and the less important ones are only run when releasing something that users will see).

[LBPHacker]

I've tested this with the clang-cl I have access to and it works fine, though I did have to disable resources because my clang-cl package didn't come with a resource compiler. I considered adding a pipeline that tests clang-cl builds but I'll put that off for now so as to not delay merging this further.